Production of caustic soda



Dee-3, 1957 H. SHAW HAL 2,815,318' 1 PRODUCTION 0F' CAUS'TIC SODA Filed July 30, 1953 7 Meeuey Guo-ra.

WEA@ pump DELNEQZJ Ou-rl-ET VVA-ree Faax?4 8 6 United States Patent() PRODUCTION F CAUSTIC SODA Harry Shaw and Arnold Lee McGavin, Runcorn, England, assignors to Imperial Chemical Industries Limited, a corporation of Great Britain Application July 30, 1953, Serial No. 371,286 Claims priority, application Great Britainy July 30, 1952 7 Claims. (Cl. 204--99) This invention relates to the production of the hydroxides of alkali metals from amalgam produced by the electrolysis of the corresponding chloride solution, using a mercury cathode and more particularly to improved methods of raising the temperature of the denuder in which the amalgam is decomposed.

Methods for the production of the hydroxide and chloride by processes involving the electrolysis of the chloride solution using a moving mercury cathode are well-known. In the production, for example, of sodium hydroxide the fluid sodium mercury amalgam formed in electrolysis is made to react with water in a separate vessel, frequently termed a denuder. In this reaction mercury is recovered and is then recycled for further use in the electrolytic cell.

In the many descriptions which have been published of this process diferent forms of denuder have been disclosed; for example, use is made in commercial practice of a trough-like vessel, provided with catalytic bodies for promoting the reaction, slightly inclined to the horizontal, through which the amalgam and the water are made to flow. Another form of denuder comprises a tower packed with catalytic bodies, over which the amalgam ows downwardly through the aqueous medium with which it is reacting. In yet another form, water passes upwards through a tower packed with catalytic bodies, in which the packing is flooded with the amalgam which is undergoing decomposition.

In all these forms of apparatus, the conditions of operation can vary within wide limits, and the highest concentration of sodium hydroxide solution which can conveniently be made can also vary within wide limits.

It is well known that other conditions being the same, the higher the temperature at which the denuder can be operated, the higher is the concentration of sodium hydroxide which can be conveniently produced or the greater is the ease with which a given concentration can be attained, or the greater is the amount of sodium hydroxide which can be produced in a denuder of specified size.

The temperature of operation can be increased by the application to the denuder, or to the materials entering it, of heat derived from some external source, but considerable additional cost is entailed in the process if steam, electricity or other fuel have to be used specifically for this purpose.

An alternative method of providing heat for maintaining a high temperature in the denuder is by the recovery and return of some of the heat generated by its own operation. The reaction between the alkali metal and the water in the denuder is exothermic, and the heat evolved in normal operation causes the temperature of the amalgam to rise during its passage through the denuder so that the mercury leaving the denuder contains more sensible heat than the amalgam entering it.

Means of this nature for the application of heat to a denuder are described in British specification No.

19,842/48 as open to public inspection. The method here adopted is to heat the relatively cool amalgam as it enters the denuder by. transferring thereto heat from,

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the relatively hot mercury leaving the denuder by means of a suitable liquid medium which is brought into indirect heat transfer relationship, rst with the hot mercury and then with the cool amalgam. There is mentioned the use of diphenyl as an example of suitable heat exchange liquid, the diphenyl being circulated by a special pump which forces it through tubular heat interchangers when a transfer of heat takes place.

We have now found that heat may be returned to the denuder in a very simple, ecient and economical manner by establishing a circulation of heat transfer liquid which is brought successively into direct contact with the hot mercury issuing from the denuder and the cold amalgam entering the denuder. Particular advantages result from the use of water or alkali metal hydroxide solution as the heat transfer liquid.

According to the present invention therefore an improvement in the manufacture of alkali metal hydroxides by rst electrolysing the corresponding metal chloride using a flowing mercury cathode and thereafter reacting the alkali metal amalgam so formed with water in a separate vessel or denuder comprises a process for transferring heat from the hot mercury issuing from the denuder to the cold amalgam entering the denuder through the medium of an aqueous uid which is brought into direct heat transfer relationship with both the hot mercury issuing from the denuder and the cold amalgam entering the denuder.

It will be readily understood that when water is used as the vehicle of heat exchange between the hot mercury and the cool amalgam, the direct contact of water and amalgam will result in the formation of a quantity of a solution of alkali metal hydroxide. This occurs even though no particular steps are taken to promote the interaction. For convenience in the ensuing description the term water will be used to imply either water or a solution of alkali metal hydroxide. Alkali metal hydroxide will also be particularly referred to as sodium hydroxide as it is in the manufacture of this compound that the invention will be most generally useful.

In an advantageous form of our invention the water, after having transferred heat to the cool amalgam, is circulated to contact more hot mercury leaving the denuder and by maintaining a rapid circulation of water in this manner, a proportion of the reaction heat absorbed by the mercury is returned to the denuder by the owing amalgam.

For practical purposes, it is desirable to minimise the quantity of sodium hydroxide which is in the circulation heat exchange liquor. In an advantageous form of the invention, therefore, We transfer continuously from the circulating heat exchange liquor the Water required for production of sodium hydroxide in the denuder and replace it from an external source.

The process of the present invention can be operated not only in the types of mercury cell plant in which both the mercury cathode in the electrolysis cell and the amalgam in the denuder flow over the base of trough-shaped vessels which are inclined to the horizontal, but also to plants in which the denuder has the form of a tower which may be packed with bodies promoting the reaction between the amalgam and water.

In both these forms of apparatus it is convenient and customary to allow the amalgam to flow through the apparatus under the inuence of gravity and to maintain the flow by continuously raising it with some form of pump or lifting mechanism from the lowest point in the system which the mercury attains back to the highest. Commonly the pump is so located that it raises the mercury from the point where it leaves the denuder to the point where it enters the electrolytic cell.

We have found, and this is to be regarded as a further feature of our invention, that it is advantageous so to elevate mercury from the denuder to the electrolytic cell that during the elevation it is brought into close contact with the water used as a circulating heat transfer liquid. l In adapting the process of our invention to practical operation, it will obviously be desirable to take all the precautions which are reasonably possible to conserve heat, but subject to such precautions, many conventional forms of plant may be used.

We have been able to adapt the invention very conveniently to the type of plant in which both the electrolytic cell and the denuder are in the form of troughlike vessels slightly inclined to the horizontal, and in which the mercury and amalgam arecausedto flow over the base of the troughs. Using such apparatus, the amalgam issuing from the electrolytic cell is caused to ow downwardly through a heat interchanger equipped with a series of inclined planes adapted to conduct the amalgam in paths of reversing directions, countercurrent to, and4 in direct contact with, water which is forced through the heat interchanger from the bottom. Advantageously, though not necessarily, the heat interchanger and planes are constructed of material having little or no tendency to promote the reaction of water with amalgam. From the bottom of the heat interchanger the heated amalgam is passed to the upper end of the denuder, in which it is reacted with water in the presence of the usual cathodic bodies to produce concentrated sodium hydroxide solution.

From the lower endof the denuder the mercury, which has now attained a higher temperature than the entering amalgam, is conducted into a well or. sump into which the water from the heat interchanger is also passed. The

mercury and water are in directcontact in this well and some transfer of heat from the mercury to the water takes place.

In order to provide further opportunity for the interchange of heat the mercury and the water collecting in the sump are simultaneouslyraised through a second heat interchanger which may itself comprise a pump suitable for raising the two liquids. Alternatively, the water and mercury may be passed countercurrent through a second heat interchanger similar in construction to the first. Yet a further possibility is to use the circulating heat transfer liquid to actuate a water lift pump for raising the mercury. During this operation, heat transfer relationship is established.

After passing through whichever of these heat interchange devices is employed, the mercury and water are separated and are returned respectively to the electrolytic cell and the first heat interchanger.

In order to obtain the fullest advantageV from our invention we supply the water required for the production of sodium hydroxide solution in the denuder from the circulating heat interchanger water and replace it in the circulating system from any convenient source. We also ind it advantageous to use such replacement water before it enters the circulating system to wash the mercury before it enters the electrolysis cell. By this step small amounts of sodium hydroxide solution which may be carried by the mercury are removed.

Although not referred to in the above description, it will, of course, be evident that all appropriate means, such as the provision of heat insulation for pipelines and other parts of the apparatus, are used to conserve heat and prevent loss to the surrounding atmosphere.

It will also be evident that it is necessary to provide at certain positions in the apparatus devices, such as, for example, lutes and weirs, to separate the mercury and water and to direct their flow correctly. Since such devices are for the most part conventional in their form and purpose, it is unnecessary for description of the invention to refer to them in detail.

The invention will now be more particularly described with reference to the attached schematic drawing which 4, illustrates one suitable method and arrangement for carrying out the invention.

Electrolysis of sodium chloride solution takes place in the electrolysis cell 1 having a owing mercury cathode flowing downwards along the length of the cell. Conventional electrical and constructural features of the cell such as the anodes and brine inlet and outlet are not indicated in the drawing. Amalgam formed flows out of 1 into the counter-current heat interchanger 2 and thence to the denuder 3 where it reacts with water entering at 4 to form sodium hydroxide solution which is run off at the upper end of the denuder 3.

Mercury and dilute sodium hydroxide liquor from the lower end of the denuder are raised by a pump 5 driven by the pump motor 12, to the upper part of the electrolysis compartment. They flow together from the pump delivery outlet 6 into a mercury chute 7, the mercury flowing into the electrolysis cell in the normal way. As it enters the electrolysis cell the water feed at 8 rst washes the mercury in the chute 7 and then ows over a Weir 9, sealed by a mercury lute at the bottom, to join the hot liquor. The water feed and hot liquor are then taken off the mercury chute by means of a pipe which protrudes above the mercury ow level and thence down an external mild steel lagged pipe to the other end of the cell, the path of the circulating hot liquor being shown by 10. The hot liquor then enters the heat interchanger 2 and flows counter-current to the strong amalgam from the electrolysis cell. From this heat interchanger an other external mild steel lagged pipe takes the circulating liquor back to the pump end 4 of the denuder along the line indicated by 11 the liquor being led under the main frame of the cell and fed into the denuder end box by way of the normal entrance for the water feed.

The heat interchanger 2 is a rectangular chest fabricated from mild steel plate and containing a number of trays alternately sloping in opposite directions, over which the strong amalgam leaving the electrolysis cell is made to ow before entering the denuder end box. Hot liquor from the external feed pipe 14 enters at the bottom of the chest opposite the amalgam outlet, into a separate end compartment, from which it Hows through the rectangular opening in the partition at a level immediately below the bottom tray and into the interchanger proper. From there the hot liquor travels countercurrent to the strong amalgam, up the trays and there to the return pipe 15 through a rectangular outlet placed near the amalgam inlet.

A device is incorporated at the lower end of each tray to stagger the amalgamweir and so allow the hot liquor to pass counter-current upwards.

Mercury lutes are situated at the amlgam inlet and outlet. The former is part of the normal lute 'which stops the brine in the dirt box from travelling down the amalgam chute into the denuder. The latter, however, is an additional lute which prevents the hot weak circulating liquor in the heat interchanger from mixing with the strong liquor in the denuder end box. Sodium hydroxide solution formed in the denuder is run off from the upper end 13 of the denuder.

What we claim is:

1. In the manufacture of alkali metal hydroxide by first electrolysing an alkali metal chloride solution using a owing mercury cathode and thereafter reacting the alkali metal amalgam, so formed, with water in a separate vessel from which mercury leaves at a temperature higher than the temperature of the alkali metal amalgam entering the same, the improvement which comprises transferring heat from the hot mercury which has left said vessel to the cold amalgam entering said vessel through the medium of an aqueous fluid which is successively brought into direct heat transfer relationship with both the hot mercury which has left said vessel and the cold amalgam entering saidvessel, said water being separated from said mercury priori to said electrolyzingstage.

2. A process as recited in claim 1 in which said aqueous fluid is a solution of sodium hydroxide.

3. In the manufacture of alkali metal hydroxide by rst electrolysing an alkali metal chloride solution using a owing mercury cathode and thereafter reacting the alkali metal amalgam, so formed, with water in a separate vessel by owing said amalgam and said water through said vessel in contact with one another with the mercury leaving said vessel at a higher temperature than the amalgam entering the same, the improvement which comprises continuously circulating outside said vessel an aqueous fluid between and in direct contact with the hot mercury leaving said vessel and the cold amalgam entering said vessel to thereby continuously and directly transfer heat from said hot mercury to said cold amalgam.

4. A process as recited in claim 3 in which the cold amalgam is contacted with said circulating aqueous uid by owing the cold amalgam in downwardly inclined paths of reversing direction, counter-current to and in direct contact with said circulating aqueous fluid.

5. A process as recited in claim 4 in which said circulating aqueous uid and said hot mercury leaving said vessel are combined in .a common pool, simultaneously raised while in direct contact and then separated.

6. A process as recited in claim 3 in which said circulating aqueous fluid is continuously supplied with water from an external source and a portion of said circulating aqueous fluid is continuously extracted and then reacted with said amalgam to produce alkali metal hydroxide.

7. A process as recited in claim 6 in which said water from an external source is first employed to Wash the mercury which is to form said flowing mercury cathode before being supplied to said circulating aqueous fluid.

References Cited in the le of this patent UNITED STATES PATENTS 2,423,351 Taylor July l, 1947 2,551,248 Deprez May l, 1951 2,631,126 Horst Mar. l0, 1953 FOREIGN PATENTS 453,517 Great Britain Sept. 14, 1936 951,159 France Apr. 11, 1949 

1. IN THE MANUFACTURE OF ALKALI METAL HYDROXIDE BY FIRST ELECTROLYSING AN ALKALI METAL CHLORIDE SOLUTION USING A FLOWING MERCURY CATHODE AND THEREAFTER REACTING THE ALKALI METAL AMALGAM, SO FORMED, WITH WATER IN A SEPARATE VESSEL FROM WHICH MERCURY LEAVES AT A TEMPERATURE HIGHER THAN THE TEMPERATURE OF THE ALKALI META AMALGAM ENTERING THE SAME, THE IMPROVEMENT WHICH COMPRISES TRANSFERRING HEAT FROM THE HOT MERCURY WHICH HAS LEFT SAID VESSEL TO THE COLD AMALGAM ENTERING SAID VESSEL THROUGH THE MEDIUM OF AN AQUEOUS FLUID WHICH IS SUCCESSIVELY BROUGHT INTO DIRECT HEAT TRANSFER RELATIONSHIP WIT BOTH THE HOT MERCURY WHICH HAS LEFT SAID VESSEL AND THE COLD AMALGAM ENTERING SAID VESSEL, SAID WATER BEING SEPARATED FROM SAID MERCURY PRIOR TO SAID ELECTROLYZING STAGE. 